This invention employs feeding at a rate slower than printing to effect conservation or saving of the transfer medium. It is believed that prior to this invention thermal transfer mediums have been universally assumed to be useful only for a single printing of a character from a ribbon area substantially identical in size to the character printed. Any significant underfeed would have been avoided as it would be expected to produce incomplete printing and possibly smearing across the width of the printhead. Accordingly, ribbon underfeed would have been avoided, especially substantial underfeed which would provide significant conservation of the ribbon.
Certain types of ribbon are commonly recognized as adapted to be overstruck during use. These are typically liquid-ink saturated fabric elements or resinous matrix elements holding liquid ink in the manner of a sponge. Ribbon feed mechanisms for such ribbon often underfeed. U.S. Pat. No. 3,528,536 to Caudill et al and 3,232,229 to Anderson are illustrative teachings of ribbon underfeed for impact printers. U.S. Pat. No. 4,132,486 to Kwan teaches a ribbon feed pattern to effect ribbon conservation, but the pattern does not involve underfeed and the printer is an impact printer. IBM Technical Disclosure Bulletin article entitled "Ribbon Drive," by D. P. Darwin, Vol. 19, No. 4, September 1976 at pp. 1407-1408 teaches thermal printing with advancing only during printing to conserve ribbon. The ribbon advance during printing is expressly said to be the same velocity as the printhead.